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The common wombat is a chubby marsupial that can weigh as much eighty pounds. It lives in an underground nest, or den, served by a network of tunnels collectively as long as a hundred feet, and all wide enough—between twelve and twenty inches—to accommodate the animal’s hind end. By midnight, it is safe to say, Peter was the only fifteen-year-old boy from Canberra testing his girth against that of a wombat.
Everything was going well until the passage through which Peter was crawling began to narrow. Because Peter’s explorations were secret, no one would know where to look for him if he turned up missing. Who would think to check a wombat hole? And the threat of burial was real. Peter had previously found the remains of wombats that had been trapped in their own tunnels. Dirt fell on his back. Deep beneath the forest, he began to worry. When tunnels cave in, animals that are effective diggers can escape. Those that aren’t, become part of the soil.
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Many burrowing animals dig with their limbs, among them the northern mole cricket of the central and eastern United States, top, and the marsupial mole of western Australia. Both have evolved particularly robust forelimbs (the animals are not drawn to the same scale).
These and illustrations that follow by Patricia J. Wynne (www.wynneart.com/) |
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Soil is built of broken-down things—mountains, trees, decayed bodies. The agents of the breakdown are the elements, and also the animals that plow through the earth. But moving through earth is not like moving through air. Earth is denser, more complexly structured, more reluctant to allow passage, and more apt to close in upon you. When people walk, air moves forward and around them. Belowground, wombats and other mammals cannot just push forward. They must dig.
The wombat, for its part, is a rather ordinary burrower. It claws at the earth with its powerful forelegs and spends part of its time aboveground. Truly subterranean animals rarely leave the earth. We only glimpse hints of their presence. We see tunnels in our lawns. We find a mole in the road, after it has tumbled out of an embankment. We sink a spade into the soil and pull up worms. We turn a stone, and a blind ant struggles to disappear. Worldwide, among mammals alone, more than 280 species in eleven families spend most of their lives underground. Still, most underground creatures are insects, worms, or other invertebrates (as are most creatures generally). When Peter looked around him in the wombat’s hole, he couldn’t help but notice other subterranean life. It must have scurried over, under, and around him.
A digging animal has a few necessities if it is to make headway. It needs to dig. And it needs to do something with the dirt it has dug—dump it out or at least compact it. Those two simple steps, with a few twists, can create complex underground structures, ranging from a few inches to many hundreds of yards in length. Almost all animals’ burrows feature two kinds of spaces, chambers and tunnels. (Earthworms mostly make tunnels; ants make lots of chambers.) On top of that basic structure, there are many additional adjustments to prevent collapse, to make it hard for a weasel (for instance) to get in, to prevent carbon dioxide from building up to toxic levels, to store food, and to dispose of waste. But the basics are dig and remove.
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The eastern hognose snake, left, has a protuberance at the front of its head that helps scrape the soil and compact it upward. Rhineura floridana, right, belongs to one of several lineages of shovel-snouted amphisbaenians, or “worm lizards,” that have independently evolved a similar technique. |
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Many unrelated lineages of animals have converged on similar body types and lifestyles that make tunneling easier. In almost every case, their adaptations involve a suite of losses (such as reductions in eyes, external ears, and girth) as well as gains (heightened senses of smell and touch, longer incisors, stouter forelimbs with longer, sharper claws). Peter Nicholson possessed none of those adaptations, but he did have a trowel. To dig his way through the tight tunnel, he hacked away at the earth with his trowel, pushed dirt down beneath his belly, and kicked it behind him.
Underground life is at least superficially unappealing. There is no light. It is hard to move. It is hard to detect and find food. Nothing comes easily. But there must also be advantages to being underground, even if it is only to escape the lumbering creatures above. Among the mammals living underground are bamboo rats, moles, marsupial moles, mole rats, pocket gophers, tuco-tucos, voles, and the like, each an independent evolutionary foray into the subterranean ecosystem.
So predictable are the adaptations for life underground, that in 1974 the zoologist Richard D. Alexander of the University of Michigan predicted the existence of a kind of social mammal not then known to exist. On the basis of his knowledge of social insects, Alexander made twelve predictions about the hypothetical mammal. It would live somewhere in Africa; it would have a morphologically distinct queen;
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Naked mole rats, whose social structure resembles that of an ant colony, sometimes form chains to move excavated soil out of a tunnel. The lead animal chips its way forward with its upper canines. |
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it would live on tubers; it would engage in “cooperative reproduction”; and so on. Alexander then went looking for the predicted creature and helped discover and study it in the semideserts of Ethiopia and Kenya. The animal is now known as the naked mole rat (Heterocephalus glaber).
Naked mole rats are best known for their behavioral adaptations—who can ignore a mammal with a queen? But they also have anatomical adaptations for their underground lifestyle. They dig by lifting their head and then bringing their incisors down on the soil in front of them. Most of the digging is done at night, accompanied by low squeaking sounds—mole rat work songs.
As the incisors chip away, the mole rat also scrapes with its forelimbs and pushes the dirt under its belly and out behind itself. Naked mole rats and other members of the genus Heterocephalus occasionally even form digging chains. One mole rat shovels dirt back to a second mole rat, which, in turn, pushes the dirt farther back to others, until the last in line expels the soil outside.
One obstacle that Peter, the wombat, the mole rat, and most other mammals quickly encounter when digging is their size. It’s not just that the bigger you are, the more you have to excavate. A wider and more muscular animal exerts more force, but beyond some optimal (and typically small) size, that greater force does not translate into more force per unit area, or pressure. An elephant can pound mightily on the ground, but the pressure its foot exerts is less than that exerted
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The northern carmine bee-eater of eastern Africa, left, and the Atlantic puffin, right, rely on their beaks to loosen soil, which they then kick out with their feet. |
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by a mole rat’s teeth. Smaller and thinner animals can even push their way through the soil with their heads. In addition, where the texture of the soil is not uniform, they can often find ways between and around roots, stones, and other hard obstacles that would block a larger animal.
But if narrower bodies are easier to shove through the messy ether of soil, why are most subterranean mammals kind of, well, plump? One of the answers may be that mammals must maintain a near-constant body temperature. Long, narrow bodies have more surface area relative to their volume than do plump bodies, and so they lose heat faster. The same holds for birds, which also maintain a near-constant body temperature. Although no birds are subterranean, several species, including kiwis, many penguins, and shearwaters, do dig burrows for nesting.
Turn that logic around, and you begin to understand what kinds of animals tunnel more easily beneath us. When animals do not have to produce their own heat, it is less energetically costly to have a tubular body. And so animals shaped like the probing ends of roots have evolved repeatedly: in snakes, in a lineage of amphibians, in no fewer than four groups of lizards, and, of course, in worms and many other invertebrates. The dark landscape of soil is populated with many miniature, elongate writhing beasts—mole crickets, sand gropers, and many kinds of tunneling larvae. Like roots, they find their way chemically. They probe the soil for soft spots. They move and leave a hollow trail behind them.
 
The Louisiana pine snake, left, loosens sandy soil with its “nose” and “hoes” it out by bending its head downward. The shield-nosed cobra, right, moves its head from side to side, scraping the soil with its flat shield. |
One major advantage of having a body shaped like a tube is that relatively little dirt must be moved out of the way to advance, and the dirt need not even be moved so much as compressed. But tubular life is not without its disadvantages. Imagine you are twenty feet below ground in a hole barely wider than your body. All around you is earth and darkness. You must move forward without limbs.
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The Saharan sand viper, top, buries itself by flexing its side, as shown in the cross section of its body, bottom, and twisting to scoop out sand. |
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So how do they do it? The pine snake loosens sandy soil at the end of the tunnel by scraping with its “nose” and then bends its head down to “hoe” the sand out of the hole. The eastern hognose snake has a small protuberance on the front of its head that acts as a small spade. The shield-nosed cobra wiggles its flat nose while moving its head from side to side. In amphisbaenians, a group of lizards, limblessness has evolved several times. Some are shovel-snouted, such as Rhineura floridana, which loosens dirt with its nose, then scoops it up with its head and presses it against the roof of the tunnel (it has an enlarged scale on the top of its head, which may facilitate that movement). Keel-headed amphisbaenians scoop dirt side to side. Round-headed amphisbaenians ram straight into the soil, but precisely how they do so has yet to be studied.
Some legless animals that move through soil do not burrow forward so much as straight down. The Sahara sand viper commonly sinks into the hot desert sands to surprise prey, avoid predators, and perhaps to regulate body temperature. Instead of tunneling with its head, however, the snake remains entirely horizontal. Its secret trick is to expand one side of the body and twist at the same time. The expanded side then acts like a kind of shovel, scooping sand out from beside and beneath the snake.
Trapped in a hole precisely their own width, many animals—including humans—would die. So even for animals that lack arms and legs, the tunnels they dig generally offer them some elbow room and legroom (for want of better terms). But some legless animals can so contract and expand their bodies, or sections of them, that they can pass through a hole substantially narrower than their most expanded girth.
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The Mexican burrowing caecilian Dermophis mexicanus is an amphibian that can fatten and shorten its body by scrunching up its spine lengthwise inside its skin. The schematic diagram shows its body in relaxed mode (a) and when shortened and pressing against the tunnel sides (b). By keeping its back end braced and straightening the forward part of its spine, it rams its head against the tunnel’s forward end like a piston (c). |
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Among such shape-shifters are some species of caecilians, which are snakelike (or, less flatteringly, wormlike) amphibians. At least one of those caecilians, Dermophis mexicanus, fattens and shortens its body by scrunching up its spine lengthwise inside its skin [see illustration left]. In essence, the animal turns its skin into a kind of second tunnel, and its skeleton presses against both skin and earthen tunnel. Then, keeping its hind end anchored against the tunnel wall, the caecilian releases its front end and throws its body forward while simultaneously compressing the soil by repeatedly raising and lowering its head [see “Biomechanics: Squeeze Play,” by Adam Summers, Natural History, September 2003]. Its skeletal muscles straighten the spine, while other muscles squeeze it into a narrower, longer shape. The combined muscular forces create a hard-driving, pistonlike digging stroke—a motion that has evolved hand in hand with a toughened nose and a thick skull.
Earthworms also push forward through holes no wider than they are, but their main digging force is lateral. Contrary to popular opinion, worms do not simply eat their way through the soil. (If they have the choice, they eat leaves, which they painstakingly pull into their tunnels from the soil surface.) To move, an earthworm pushes the front of its body into whatever crack is before it and then expands its body laterally by shortening itself. The expansion widens the hole and opens up additional cracks it can follow. The technique is known as “crevice burrowing.” It is perhaps the best animal imitation of the narrow end of a digging plant root. Like a knife or a sharp stick, small crevice-burrowing worms can exert much more pressure against the soil than big worms can—hence the relative rarity of the latter.
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The night crawler first wriggles into an existing crevice and then expands laterally (detail), compacting the soil around it. |
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Recently Kelly M. Dorgan, a graduate student in oceanography at the Darling Marine Center of the University of Maine, in Walpole, Maine, reported yet another way of moving forward. Dorgan put marine worms in a kind of transparent gelatin, at about the same density as sediment, so that she could watch how they moved. She and her co-workers also flooded the gelatin with light; the force the burrowing worm exerted on the gelatin at various points along its body caused differences in how the light was reflected. Dorgan and her team were able to quantify those differences, thereby getting a good picture of how the creature burrows.
Their results show that the marine worm moves forward by extending its mouth—or technically, its pharynx. The animal extrudes its pharynx into a crack, exerting the greatest force at the crack’s rim. The crack widens the way split wood spreads under the force of a wedge.
For all of the organisms I’ve discussed so far, the soil is a barrier that must be carved away, chewed up, or pushed to the side. But for the smallest organisms, the soil is an ether. As solid as it seems, soil is between 40 and 60 percent air. Small animals, such as mites and springtails, simply pass from air pocket to air pocket. Ants and termites are perhaps the smallest organisms that still dig, excavating dirt by the mouthful. Some kinds of termites and ants (including army ants) live their entire lives underground. Not coincidentally, those species lack eyes and are narrow-bodied. Army ants and subterranean termites can dig, but more often than not they just pass belowground among air pockets too small for us to notice.
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African termites of the species Macrotermes natalensis carry soil particles to the surface in their mandibles. There they cement the particles into an ever-growing mound. |
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As Peter Nicholson’s wombat tunnel narrowed and its collapse became ever more likely, the young explorer might well have been alarmed. But discovery beckoned more insistently than reason. Determined to push on, he struggled through the narrow part of the tunnel and went even deeper before coming back to the surface. There, he carefully noted and sketched what he had seen.
Peter made many other forays underground, at times even coming face-to-face with the wombats. The result, the most complete study of the burrows of common wombats that had ever been done, earned first prize as a science project. His story is true; it is told in James Woodford’s delightful book, The Secret Life of Wombats.
Peter eventually graduated from his boarding school and went on to become a businessman. But—a true amateur—he never tired of watching animals. When Woodford was writing The Secret Life of Wombats, Peter welcomed the chance to go back to see the creatures. Within minutes, he began to move some dirt and lower himself into a hole. He made it to the first turn in the tunnel but then had to come back up. Like Alice after her visit to Wonderland, Peter had grown too big. His secret world had sealed shut, as though all of it had been a dream.
 An assistant professor of zoology at North Carolina State University in Raleigh, Robert R. Dunn says he became interested in burrowing animals while digging for ants with a spade, because it led him to appreciate the seeming ease with which some creatures move through the soil. A frequent contributor to Natural History, Dunn’s earlier article about army ants, “Impostor in the Nest” (June 2003), was based on a field season chasing army ants as they poured into and out of the soil. “I spent many days,” he recalls, “wishing I had the forearms of a mole or were the size and shape of a worm.” Another focus of his work is the evolutionary, ecological, and biogeographic consequences of dispersal of seeds by ants, the subject of his most recent article for this magazine, “Jaws of Life” (September 2005).
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Copyright © Natural History Magazine, Inc., 2006
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eter J. Nicholson sneaked out of his Australian boarding school bedroom one night and ran into the nearby forest. The moon was slight and the clouds were heavy, but he knew where he was going. Once there, he took out his flashlight and lowered himself headfirst into the burrow of a common wombat.